Title: A laboratory manual for comparative vertebrate anatomy
Identifier: cu31924021952902 (find matches)
Year: 1922 (1920s)
Authors: Hyman, Libbie Henrietta, 1888-1969
Subjects: Anatomy, Comparative
Publisher: Chicago, Ill. : University of Chicago Press
Contributing Library: Cornell University Library
Digitizing Sponsor: MSN

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COMPARATIVE ANATOMY OF THE UROGENITAL SYSTEM 279 as a vagina to receive the penis. In mammals above marsupials the two vaginae fuse to a single vagina (hence the name Monodelphia). There is also generally more or less fusion of the two uteri (Fig. 63). When only the posterior portions of uteri are fused, the fused portion is called the body of the uterus and the separate portions the horns of the uterus. In man and other primates the uteri are fused along their entire length producing the single uterus or womb. The young of the placental mammals develop only in the uterine part of the oviducts • in those forms with partially fused uteri, only in the horns. 5. The evolution of the cloaca.—The cloaca is found in all vertebrates except cyclostomes, teleostomes, and the placental mammals. It receives the termination of the intestine and the urinary and genital ducts. From the preceding account it will be evident that in the males of elasmobranchs and Amphibia the cloaca receives only the Wolffian ducts, while in the females both oviducts and Wolffian ducts enter it (Fig. 62A and E). It commonly happens however, that in the males of these groups vestiges of the oviducts are present. In the males
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Fig. 64.—Diagrams to illustrate the changes in the cloaca in mammals during development. A, early embryonic stage, showing the cloaca receiving the urinary bladder, the rectum, and the Wolffian duct, as in the lower vertebrates. B, later stage, showing the beginning of the fold which divides the cloaca into a ventral urogenital sinus which receives the urinary bladder, Wolffian ducts, and ureters, and into a dorsal part which receives the rectum. C, further progress of the fold, dividing the cloaca into urogenital sinus and rectum; the ureter has separated from the Wolffian duct and is shifting anteriorly. D, completion of the fold, showing complete separation of the cloaca into ventral uro- genital sinus and dorsal rectum. Note in D that the ureter has shifted farther so that it opens into the urinary bladder. of reptiles and birds the cloaca receives the Wolffian ducts (vasa deferentia) and the ureters; in the females the oviducts and the ureters (Fig. 62C and D). In addition, in many fishes, Amphibia, reptiles, and the embryos of birds and mammals the urinary bladder opens into the ventral wall of the cloaca. Adult birds have no urinary bladder; mammals have one, but it is no longer attached to the digestive tract. In placental mammals marked changes occur in the relations of the terminal portions of the urogenital 'ducts. In the embryo the cloaca becomes divided by a fold which extends posteriorly to the body wall and separates the cloaca into two parts, each with its own opening to the exterior (Fig. 64). The dorsal part includes the intestine only; this terminal portion of the intestine is called the rectum and opens to the exterior by the anus. The ventral part separated from the cloaca is called the urogenital sinus. It receives the stalk of the bladder and the excretory and genital ducts. The excretory ducts (either Wolffian ducts or ureters) at first open into the urogenital sinus, but subsequently the ureters shift so as to open into the blad - der, in all of the placental mammals (Fig. 64). Thus, the ureters pass into the bladder while the Wolffian ducts (vasa deferentia) in males or the vagina in females unite with the duct of the bladder, named the urethra, forming a common tube or chamber, the urogenital sinus, which opens externally in front of the anus by a urogenital aperture. In the females only of the

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